I'm trying to import users and mailboxes from a ZCS 6.0.1 FOSS to ZCS 7.0.0 FOSS.

Importing users went ok, but when it came to importing the mailboxes it fails.
I figure it has something to do with the certificate, but what and how do I fix it? Can anyone point me in the right direction?

If you get this kind of error with IMAP logins to external mailboxes, you need to import the CA crt or pem file from those IMAP servers. Beware imap servers on various flavours of linux that dont have commercial keys (which is what would cause this problem of course - most commercial cert roots are in zimbra already of course) - the openssl-snakeoil keys generally are generated without a CA (or without a CA file around to copy to zimbra). Regenerating your own CA and SSL key off it, then using those in your imap servers (dovecot, etc) by editing their configs, and copying the CA file to zimbra and importing it works.